In the extensive development work which has been performed on nickel base superalloys, virtually every possible element has been evaluated as an addition. It does not appear that manganese has ever been observed to have a beneficial result on superalloy properties. Manganese is often mentioned in superalloy patents but only as an impurity. Some work has been reported on the use of lanthanum and manganese mixtures for improved high temperature oxidation performance. This work is summarized in the report "Nickel-Base Superalloy Oxidation" by G. E. Wasielewski et al of General Electric, AFML-TR-69-27, Feb. 1969, pps. Lanthanum is indicated as promoting the formation of manganese-chromium spinels.
The review articles "Impurities and Trace Elements in "Nickel-Base Superalloys" by R. T. Holt et al, Int. Metal Rev., March 1976, pps 1-24, indicates that manganese is generally a detrimental trace element but that it may be added to reduce the sulfur content of nickel alloys.